Biologically fixed N_2 as a source for N_2O production in a grass-clover mixture,measured by ~(15)N_2

摘要

The contribution of biologically fixed dinitrogen(N_2)to the nitrous oxide(N_2O)production in grasslands is unknown.To assess the contribution of recently fixed N_2 as a source of N_2O and the transfer of fixed N from clover to companion grass,mixtures of white clover and perennial ryegrass were incubated for 14 days in a growth cabinet with a ~(15)N_2-enriched atmosphere(0.4 atom% excess).Immediately after labelling,half of the grass-clover pots were sampled for N_2 fixation determination,whereas the remaining half were examined for emission of ~(15)N labelled N_2O for another 8 days using a static chamber method.Biological N_2 fixation measured in grass-clover shoots and roots as well as in soil constituted 342,38 and 67 mg N m~(-2)d~(-1)at 16,26 and 36 weeks after emergence,respectively.The drop in N_2 fixation was most likely due to a severe aphid attack on the clover component.Transfer of recently fixed N from clover to companion grass was detected at 26 and 36 weeks after emergence and amounted to 0.7 +- 0.1 mg N m~(-2)d~(-1),which represented 1.7 +- 0.3% of the N accumulated in grass shoots during the labelling period.Total N_2O emission was 91,416 and 259 mu g N_2O-N m~(-2)d~(-1)at 16,26 and 36 weeks after emer-gence,respectively.Only 3.2 +- 0.5 ppm of the recently fixed N_2 was emitted as N_2O on a daily basis,which accounted for 2.1 +- 0.5% of the total N_2O-N emission.Thus,recently fixed N released via easily degradable clover residues appears to be a minor source of N_2O.